Bug: SDK crashes when using special characters in prompt

## Description

When using the Copilot SDK with special characters in the prompt, the SDK throws an unhandled exception.

## Steps to Reproduce

1. Install copilot-sdk v1.0.60
2. Create a simple test script
3. Pass a prompt containing unicode characters

## Expected Behavior

The SDK should handle special characters gracefully.

## Actual Behavior

Crashes with `TypeError: Cannot read properties of undefined`

## Environment

- OS: Ubuntu 22.04
- Node.js: 22.x
- SDK Version: 1.0.60

No source file or test is named. Start by reproducing the crash with copilot-sdk v1.0.60 on Node.js 22.x using a minimal script and a prompt containing unicode characters, then trace the prompt-handling entry point. Done means the same prompt no longer produces the unhandled TypeError and a regression test covers it.

Written by the indexing model from the issue text.